1. Field of the Invention
The present invention relates to a steering force control device of a power steering system in which a steering assistance force and a steering reaction force are generated in correspondence with the steering torque acting on an input shaft for steering a vehicle, and a physical quantity factor, which varies depending on a travelling condition of the vehicle and on a steering condition, is detected, and steering feeling is varied in accordance with a change in the physical quantity. In particular, the present invention relates to a steering force control device of a power steering system in which the appropriate steering feeling is made by changing any one of the steering assistance force, steering reaction force, and steering angle ratio in accordance with the physical quantity factor.
2. Description of the Prior Art
In prior art power steering systems, generally, there are relatively no problems at low vehicle speeds. However, problems are encountered in high speed running, or at a large steering angle in that the driver tends to become uneasy since the steering force is too light even when a physical factor, which depends upon the vehicle running condition and the steering condition, is increasing. Power steering systems which have been developed to solve this problem attempt to vary the input and output characteristics (relationships between various factors and an input shaft steering force) in accordance with the physical factors. These systems have been generally divided into the following types, including: a flow rate control type as disclosed, for example, in Japanese Patent Publication No. 54-5571 (1979), wherein a flow rate of fluid supplied to the steering system is controlled in accordance with the physical quantity factor; a steering reaction force control type as disclosed, for example in Japanese Patent Publication No. 49-29653 (1974), wherein a reactive hydraulic pressure produced by detecting the physical quantity factor is made to act directly on the input shaft as a control force such as a couple of forces; and a steering angle ratio control type as disclosed, for example, in U.S. Pat. No. 4,310,063 wherein a variable steering angle ratio is obtained by interposing resilient means which provides a rotational displacement in a range of a substantially linear spring constant.
Further, in a flow rate control type of power steering system wherein the flow rate of the fluid is controlled by actuating an electric-hydraulic pressure converter in accordance with the detected physical quantity factor, there is a problem in that, when an electromagnetic solenoid is used as the electric-hydraulic pressure converter, the flow rate of the fluid is varied depending on a temperature change of the electromagnetic solenoid, and thus proper steering failure cannot be obtained.
In order to solve this problem, for example, as described in Japanese Patent Publication Nos. 54-11047 and 55-4394, an electrical type of control has been proposed to obtain an adequate steering reactive hydraulic pressure by adjusting an energizing current supply rate to the electric-hydraulic pressure converter in accordance with a control signal corresponding to a vehicle speed. In this case, a voltage drop or a load current in the electric-hydraulic pressure converter is detected, and this detected signal is supplied as a feedback signal to a differential amplifier to which a predetermined reference voltage or a vehicle speed detection signal is being supplied, thereby to eliminate the influence due to a large variation in the internal resistance of the electric-hydraulic pressure converter caused by the temperature change, or due to a variation in the power supply voltage.
However, in the prior art flow rate control type, although there is an advantage that a control mechanism is relatively simple and is easily applicable to other usual power steering systems, a drawback is also present in that the steering force of the input shaft is not increased to a great extent even when the flow rate is varied in accordance with a variation in the physical quantity factor.
Furthermore, in the steering reaction force control type, a range of the input and output characteristics can be made wide, however, on the other hand, there is a problem in that since it is required to introduce a reactive hydraulic pressure from outside and further to convert this hydraulic pressure to a direct control force, the structure tends to be complicated. Moreover, since a special pressure is applied to the input shaft for controlling a normal steering force at the time of high speed running or at the time when the physical quantity factor is increased, a variation in the pressure is abrupt, and additional energy is consumed due to this variation, which poses a problem in view of the efficiency of the fuel consumption.
In addition, in the steering angle control type, in order to make the steering angle ratio large, it is necessary to set the spring constant of a torsion bar low to permit a large torsional angle at the resilient means. As a result, there arises a problem in that the torsional rigidity of the torsion bar is reduced, and the steering feeling for the steering system as a whole is degraded.
Further, the electrical control type is constructed to control, in accordance with the magnitude of a current, the electromagnetic force of the electromagnetic solenoid in the electric-hydraulic pressure converter which generates the hydraulic pressure corresponding to the vehicle speed. In this type of control, when the vehicle is running the current is consistently supplied to the electromagnetic solenoid, and heat generation in an output transistor which performs the control is increased, and thus there is a problem in that the reliability of semiconductor device is degraded and a large heatsink is required. As a measure to minimize this drawback, the heat generation of the output transistor is reduced by making the output transistor perform a switching operation and by changing the duty ratio of this transistor in accordance with the vehicle speed. However, in this case, since a pulse width applied to the electromagnetic solenoid is controlled, the amount of the energizing current of the electromagnetic solenoid cannot be detected accurately, and thus it is impossible to modify the control by feeding back the voltage drop caused by the current flowing through the electromagnetic solenoid as in the prior art mentioned above. As a result, there arises a new problem in that it is impossible to prevent the variation in the amount of energizing current due to an internal resistance change which is caused by the temperature change of the electromagnetic solenoid.